1. Field of the Invention
The present invention relates to a self light emitting device, and more particularly, to an active matrix self light emitting device. Among such devices, in particular, the present invention relates to an active matrix self light emitting device using self light emitting elements, such as organic electroluminescence (EL) elements, in a pixel portion.
2. Description of the Related Art
The spread of self light emitting devices in which a semiconductor thin film is formed on an insulator such as a glass substrate, in particular to active matrix self light emitting devices using thin film transistors (hereafter referred to as TFTs), has been remarkable recently. Active matrix self light emitting devices have from several hundred thousand to several million TFTs formed in a matrix shape in a pixel portion, and display of an image is performed by controlling the electric charge of each of the pixels.
In addition, techniques relating to polysilicon TFTs used for simultaneously forming driver circuits using TFTs formed in the periphery of the pixel portion, in addition to pixel TFTs structuring the pixels, have been developed recently, and these contribute greatly to the miniaturization of devices, and also to reducing the electric power consumption of the devices. Self light emitting devices have thus become indispensable devices in display portions of mobile devices having a remarkably wide range of applications in recent years.
Further, self light emitting devices which apply self light emitting materials such as organic EL materials as flat displays in substitute for LCDs (liquid crystal displays) are under the spotlight, and are being enthusiastically researched.
A schematic diagram of a normal self light emitting device is shown in FIG. 15A. The use of an organic EL element (hereafter referred to simply as an EL element) as one example of a self light emitting element is explained in this specification. A pixel portion 1504 is arranged in the center of an insulating substrate (such as glass, for example) 1501. In addition to source signal lines and gate signal lines, electric current supply lines 1505 for supplying electric current to EL elements are arranged in the pixel portion 1504. A source signal line driver circuit 1502 for controlling the source signal lines is arranged on the top side of the pixel portion 1504, and gate signal line driver circuits 1503 are placed on the left and the right of the pixel portion 1504 in order to control the gate signal lines. Note that although the gate signal line driver circuits 1503 are arranged on both the left and right sides of the pixel portion in FIG. 15A, they may also both be placed on the same side. However, from the perspectives of drive efficiency and reliability, it is preferable to arrange the gate signal lines on both sides. Input of signals from the outside into the source signal line driver circuit 1502 and the gate signal line driver circuits 1503 is performed via a flexible printed circuit (FPC) 1506.
An expanded view of a portion surrounded by a dotted line frame 1500 within FIG. 15A is shown in FIG. 15B. The pixel portion has pixels arranged in a matrix shape, as shown in the figure. A portion additionally surrounded by a dotted line frame 1510 within FIG. 15B is one pixel, and the pixel has a source signal line 1511, a gate signal line 1512, an electric current supply line 1513, a switching TFT 1514, an EL driver TFT 1515, a storage capacitor 1516, and an EL element 1517.
Operation of active matrix self light emitting devices is explained next while referring to the same FIG. 15B. First, a voltage is applied to the gate electrode of the switching TFT 1514 when the gate signal line 1512 is selected, and the switching TFT 1514 is placed in a conductive state. The signal (voltage signal) of the source signal line 1511 is stored as an electric charge in the storage capacitor 1516 by doing so. A voltage VGS between a gate and a source of the EL driver TFT 1515 is determined by the electric charge accumulated in the storage capacitor 1516, and an electric current corresponding to the voltage of the storage capacitor 1516 flows in the EL driver TFT 1515 and in the EL element 1517. The EL element 1517 turns on as a result.
The brightness of the EL element 1517, equal to the amount of electric current flowing in the EL element 1517, can be controlled in accordance with VGS of the EL driver TFT 1515. VGS is the voltage of the storage capacitor 1516, and that is the signal (voltage) input to the source signal line 1511. In other words, the brightness of the EL element 1517 is controlled by controlling the signal (voltage) input to the source signal line 1511. Finally, the gate signal line 1512 is placed in an unselected state, the gate of the switching TFT 1514 is closed, and the switching TFT 1514 is placed in an unselected state. The electric charge which has accumulated in the storage capacitor 1516 is maintained at this point. VGS of the EL driver TFT 1515 is therefore maintained as is, and the amount of electric current corresponding to VGS continues to flow in the EL element 1517 via the EL driver TFT 1515.
Information regarding EL element drive is reported upon in papers such as the following: Current Status and Future of Light Emitting Polymer Display Driven by Poly-Si TFT, SID99 Digest, p. 372; High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver, ASIA DISPLAY 98, p. 217; and 3.8 Green OLED with Low Temperature Poly-Si TFT, Euro Display 99 Late News, p. 27.
A method of gray scale display in the EL element 1517 is discussed next. An analog gray scale method for controlling the brightness of the EL elements 1517 by the voltage VGS between the gate and the source of the EL driver TFT 1515 has a disadvantage in that it is weak with respect to dispersion in the electric current characteristics of the EL driver TFTs 1515. That is, if the electric current characteristics of the EL driver TFTs 1515 differ, then the value of the electric current flowing in the EL driver TFTs 1515 and the EL elements 1517 changes even if the same gate voltages are applied. As a result, the brightnesses of the EL elements 1517, namely the gray scales, also change.
A method referred to as a digital gray scale method has therefore been proposed in order to reduce the influence of dispersion in the characteristics of the EL driver TFTs 1515 and obtain a uniform screen picture. This method is a method for controlling the gray scale by two states, a state in which the absolute value |VGS| between a gate and a source of the EL driver TFT 1515 is below the turn on start voltage (in which almost no electric current flows), and a state in which the absolute value |VGS| is greater than the brightness saturation voltage (in which an electric current close to the maximum flows). In this case, the value of the electric current becomes close to IMAX even if there are dispersions in the electric current characteristics of the EL driver TFTs 1515, provided that the absolute values |VGS| of the EL driver TFTs 1515 are sufficiently larger than the brightness saturation voltage. The influence of EL driver TFT dispersions can therefore be made extremely small. The gray scales are thus controlled by two states, an ON state (bright state due to maximum electric current flow) and an OFF state (dark state due to no electric current flow). This method is therefore referred to as a digital gray scale method.
However, only two gray scales can be displayed with the digital gray scale method. A plurality of techniques which can achieve multiple gray scales, in which another method is combined with the digital gray scale method, have been proposed.
A time gray scale method is one method which can be used to achieve multiple gray scales. The time gray scale method is a method in which the time during which the EL elements 1517 are turned on is controlled, and gray scales are output by the length of the turn on time. In other words, one frame period is divided into a plurality of subframe periods, and gray scales are realized by controlling the number and the length of the subframe periods during which turn on is performed.
Refer to FIGS. 9A and 9B. Simple timing charts for a time gray scale method are shown in FIGS. 9A and 9B. An example of obtaining 3-bit gray scales by a time gray scale method with the frame frequency set to 60 Hz is shown.
As shown in FIG. 9A, one frame period is divided into a number of subframe periods corresponding to the number of gray scale bits. Three bits are used here, and therefore one frame period is divided into three subframe periods SF1 to SF3. One subframe period is further divided into an address period (Ta#) and a sustain (turn on) period (Ts#). (See FIG. 20B.) A sustain period during a subframe period denoted by reference symbol SF1 is referred to as Ts1. Similarly, sustain periods for the cases of subframes SF2 and SF3 are referred to as Ts2 and Ts3, respectively. Address periods Ta1 to Ta3 are each periods during which one frame portion of an image signal is written into the pixels, and their lengths are therefore equal in all of the subframe periods. The sustain periods have lengths proportional to powers of 2, and the sustain periods here are such that Ts1:Ts2:Ts3=22:21:20=4:2:1.
As a gray scale display method, the brightness is controlled by the sum of all the sustain (turn on) periods within one frame period in accordance with controlling which subframe periods the EL elements are turned on, and which subframe periods the EL elements are not turned on, in the sustain (turn on) periods from Ts1 to Ts3. In this example, 23=8 turn on time lengths can be set by combining the sustain (turn on) periods, and therefore 8 gray scales from 0 (all black display) to 7 (all white display) can be displayed, as shown in FIG. 9B. Gray scales are thus expressed by utilizing the length of the turn on time. Similar gray scale expression is also possible, of course, in a color display self light emitting device.
In addition, the number of divisions within one frame period may also be increased for a case of increased gray scales. The proportional lengths of the sustain (turn on) periods for a case of dividing one frame period into n subframe periods become Ts1:Ts2: . . . :Ts(n-1):Tsn=2(n-1): 2(n-2): . . . :21:20, and it becomes possible to express 2n gray scales. Note that the appearance of the subframe periods may be in random order from SF1 to SFn. Note also that gray scale expression is possible even if the lengths of the sustain (turn on) periods are not made into powers of two.
Problem points relating to self light emitting devices using self light emitting elements such as EL elements are discussed. As stated above, electric current is always supplied during the periods in which the EL elements are turned on, and the electric current flows within the EL elements. The nature of the EL elements degrades due to being turned on for a long time, and the brightness characteristics change with this as a cause. That is, even if the same electric current at the same voltage is supplied from the same electric current supply source, a difference develops between the brightness of EL elements which have degraded and EL elements which have not degraded.
A specific example is explained. FIG. 10A is a display screen of a device using a self light emitting device, such as a portable information terminal and icons for operation and the like 1001 are displayed. The proportion of time during which there is static display like that shown in FIG. 10A is normally large with this type of portable information device. If icons and the like are displayed by a color (gray scale) which is brighter than the background, then the EL elements in the pixels of portions which display the icons and the like are turned on for a longer time that the EL elements of portions displaying the background, and degradation proceeds very quickly.
The EL elements are assumed to have been degraded by these conditions. Display examples of the self light emitting device after degradation are shown in FIGS. 10B and 10C. First, for a case of black display such as that shown in FIG. 10B, the self light emitting elements such as EL elements are in a state in which a voltage is not applied, namely the EL elements express black by not turning on, and therefore the degradation is not a problem for black display. However, for a case of white display, even if the same electric current is supplied to the EL elements which have been degraded due to long turn on time (EL elements of portions displaying icons and the like in this case), there is insufficient brightness and irregularities develop as shown by reference numeral 1011 in FIG. 10C.
There is a method in which the voltage applied to the degraded EL elements is raised in order to eliminate the brightness irregularities, but the electric current supply line in a self light emitting device is normally structured by a single wiring. Further, it is not easy to structure a circuit for changing the voltage applied to the EL element in one specific pixel within the pixels arranged in a matrix shape of a pixel portion. In addition, there is dispersion of the EL driver TFTs, as stated above, and this correction method cannot be said to be preferable.
There is a technique recorded in Japanese Patent Application Serial No. 2000-273139 as a method of solving the above problem points. A simple explanation of this technique is made using FIG. 18.
FIG. 18 is a schematic diagram of a device in a self light emitting device having a degradation correction function recorded in Japanese Patent Application Serial No. 2000-273139. In accordance with this method, the turn on time of each pixel, or the turn on time and the turn on strength, is detected by periodically sampling a first image signal 1801A using a counter 1802 and stored in memories 1803 and 1804. The sum of the detected values, and the hourly change data of the brightness characteristics of the EL elements already stored in a correction data storage portion 1806 are referenced, the image signal for driving the pixels having degraded EL elements is corrected by computations in a correction circuit 1805, and a second image signal 1801B is obtained. Image display is performed using the second image signal 1801B. The brightness irregularities in the display device 1807 having degraded EL elements in a portion of the pixels are thus corrected, and a uniform screen picture is obtained.
However, the degradation state of the EL elements at a certain point is not directly detected with the method stated above, and the degradation state is merely estimated from the total turn on time of the elements, or from the total turn on time and turn on strength. The term turn on strength used here is not the turn on strength of the EL elements themselves, but is obtained from reading the gray scale of the input digital image signal. There is a disadvantage in that the correction of the image signal is performed in accordance with correction data prepared in advance; in other words, degradation not due to driving time cannot be dealt with. For example, reductions in brightness developing from degradation due to temperature changes or the like cannot be coped with by the count of only the total turn on time. There also cannot be a response to brightness defects due to dispersion in the initial properties of the elements themselves with this method.